This invention relates to a connecting ring for Coriolis flowmeter and in particular to a method and apparatus that enables the bonding of Coriolis flowmeter elements having different thermal coefficients of expansion.
Single tube Coriolis flowmeters typically have a balance bar surrounding a flow tube and intermediate annular connecting rings that couple each end of the balance bar to the flow tube. The connecting rings are often affixed to the balance bar and the flow tube by a brazing process in order to provide a rigid and permanent connection. The integrity of the braze joints is important because, in operation, the balance bar and the material filled flow tube are vibrated in phase opposition. The flow tube vibration is necessary to produce the Coriolis acceleration on the flowing material and the balance bar vibration is necessary to counterbalance the vibrating flow tube. The connecting rings and their braze joints ensure that the flow tube, the connecting rings and the balance bar define an integral dynamically balanced structure. If the joints are not of high and consistent integrity, the balance of the vibrating structure can be impaired along with the accuracy of the flowmeter.
Flaws in the braze joints can also reduce the life of a flowmeter. The location of the joints between the oppositely vibrating balance bar and material filled flow tube puts the braze joints in a region of high stress. Furthermore, the stress is cyclic and reverses sign with every vibration cycle. Flawed or incomplete braze joints tend to have geometries which concentrate and increase the cyclic stress. The stress can even be elevated to the point where it causes fatigue cracking and failure of the meter. It can thus be seen that the connecting rings and the brazes constitute critical elements in the successful operation of a Coriolis flowmeter.
Prior art meters have traditionally been brazed by an operation in which the cylindrical balance bar is placed over the flow tube and then the annular connecting rings are placed over the flow tube and into the ends of the balance bar. Braze material is applied to the surfaces that couple the connecting ring to the balance bar and flow tube. The structure is then placed in a oven and heated to approximately 800xc2x0 C. The braze material melts and flows by capillary attraction into the small clearances separating the flow tube, connecting ring, and balance bar. The structure is then cooled and braze material solidifies to form an integral structure comprising the balance bar, connecting ring, and flow tube.
The brazing process is well suited for applications in which similar material is used for the flow tube, connecting ring, and balance bar. These elements are machined prior to the brazing operation so that an optimum clearance (gap) of approximately 0.005 cm exist between the surfaces to be bonded. This gap is sufficiently small that the capillary attraction overcomes the force of gravity and sucks the liquid braze material into the gap rather than allowing it to run down the flow tube and balance bar. Upon cooling an integral solid structure is formed.
The braze process using the prior art component design, however, is not well suited for the bonding of materials having different thermal coefficients of expansion. This is a problem because it is necessary to make the flow tube of titanium for performance reasons. Titanium is very expensive and difficult to weld and fabricate. Therefore, for reasons of economy, a stainless steel balance bar is preferred. Stainless steel has a thermal expansion coefficient that is approximately twice that of titanium. When the components are heated in the brazing furnace the stainless steel balance bar expands twice as much as the titanium flow tube and connection rings. At brazing temperature this differential expansion opens the gaps between the titanium and stainless steel parts so that the capillary attraction is no longer sufficient to hold the braze material in the gaps.
For an example, let it be assumed that the parts are machined to have a 0.005 cm gap at room temperature. At brazing temperature (800xc2x0 C.) the gap between the outside of the titanium flow tube and the inside of the titanium connecting ring does not change significantly because they both expand the same amount. However, the gap between the outside of the titanium connecting ring and the inside of the stainless steel balance bar increases at brazing temperature. Titanium expands at approximately 7.2xc3x9710xe2x88x926 cm/cm/xc2x0 C. while stainless steel expands at approximately 16.2xc3x9710xe2x88x926 cm/cm/xc2x0 C. The difference in expansion rate is thus 9xc3x97106 cm/cm/xc2x0 C. Assuming that the cylindrical surfaces to be brazed have a diameter of 2.54 cm, when the structure is heated to the brazing temperature of 800xc2x0 C, the inside surface of the balance bar expands 0.0177 cm more than the outside surface of the connecting ring. The gap produced by the differential expansion is added to the original clearance of 0.005 cm to produce a gap of 0.023 cm at brazing temperature. This 0.023 cm gap is not suitable for a successful brazing operation since the capillary attraction is not sufficiently strong to prevent the liquid braze material from running out of the joint. Furthermore, if the parts are not fixtured with extreme precision, the gap is likely to become 0.046 cm on one side and zero on the other as the connecting ring moves to one side in the balance bar bore. This lack of concentricity can result in a braze that extends only partly around the circumference of the intended braze surface. The result is a defective braze joint between the flow tube and connecting ring when the structure is cooled.
Attempts have been made in the prior art to overcome the problem of brazing a titanium flow tube and to a non-titanium balance bar. These efforts include the use of threaded braze surfaces to couple the elements together. This is not satisfactory since threading of the parts is expensive and the materials still expand at different rates so that outer threads on the connecting ring would not be tightly coupled to inner threads on the balance bar resulting in loss of concentricity and the possibility of partial brazes.
For the above and other reasons it is a problem in the art of Coriolis flowmeter construction to reliably and inexpensively braze materials having different thermal expansion coefficients. In particular it is difficult to provide an integral structure wherein non-titanium balance bars are reliably brazed to titanium flow tubes and titanium connecting rings. In the above discussion it is assumed that the flow tube and connecting rings are titanium and that the balance bar is made of material such as stainless steel or other material having a higher thermal coefficient of expansion. A similar problem arises when the flow tube is made of titanium and the connecting rings and balance bar are made of stainless steel or when any of the parts to be brazed has a thermal expansion coefficient different than any other of the parts.
The above and other problems are solved and an advance in the art is achieved by a method and apparatus provided by the present invention. The present invention relates to a Coriolis flowmeter that has a geometry such that the connecting ring can be inexpensively and reliably brazed to a flow tube and balance bar of dissimilar materials. In a typical application of the apparatus and method of the present invention, a titanium connecting ring is brazed to a balance bar formed out of material having a much higher coefficient of expansion such as stainless steel.
The titanium connecting ring has a radially inner braze joint with the titanium flow tube. The connecting ring surface of this joint is axially parallel to the outer surface of the flow tube. Put simply, the inner braze surface of the connecting ring and the outer braze surface of the flow tube are cylindrical as in the prior art. Since the flow tube and the connecting ring are both formed of titanium, they have the same expansion coefficient so that the braze clearance or gap between them does not change with temperature. The stainless steel balance bar, however, has a much higher thermal expansion coefficient. To accommodate the difference in expansion coefficients, a tapered outer surface of the connecting ring mates with a matching tapered inner surface of the balance bar. The tapers of both parts are of the same angle. Both parts have the decreasing radius towards the axial center of the flow tube.
The assembly of the parts to be brazed may be oriented with the flow tube axis vertical in the braze furnace. The assembly is heated during the brazing process, and the stainless steel balance bar expands away from the titanium connecting rings. The top connecting ring drops under the force of gravity or other forces and moves downwards towards the axial center of the flow tube. The axial movement of the ring with respect to the balance bar results in the tapered connecting ring nesting deeper into the internal taper of the balance bar. This movement keeps the braze gap between the outer surface of the connecting ring and the inner surface of the balance bar negligible and thus keeps the capillary force sufficiently strong to keep the liquid braze material in the braze joint. The axial movement also keeps the connecting ring concentric with the balance bar.
The bottom connecting ring of the vertical assembly also has an external taper that is made to move into the bottom balance bar taper in a similar manner. The braze assembly can be supported in the furnace by the bottom connecting ring. This causes the weight of the balance bar and top connecting ring to push the bottom connecting ring further into the bottom balance bar taper as the balance bar expands.
In a first possible exemplary embodiment, the design of the taper defines the amount that the tapered connecting rings can axially enter into the balance bar as the assembly is heated. A smaller taper angle results in the connecting rings moving further in the axial direction in order to minimize the gap. The amount of insertion is critical because it determines the active length of the flow tube and thus the frequency, balance, and sensitivity of the meter. This embodiment requires precise machining of the matching tapers. The taper angles are kept small because the connecting ring has a small radial thickness compared to its length. The small taper angle means that small changes in taper diameter (machining tolerances) result in large changes in the axial location of the connecting ring.
In accordance with a second possible exemplary embodiment, the balance bar and the connecting rings have tapered braze surfaces as in the first embodiment. In the second embodiment, each inner braze surface of the balance bar has a machined step at its axial inner end. These steps limit the axial travel of the connecting rings into the balance bar at brazing temperatures and provide for a precise length of the active portion of the flow tube. As in the first embodiment, the balance bar expands with heating more than the connecting rings. The connecting rings then move into the balance bar internal tapers. The rings are designed to abut against the balance bar steps before the assembly reaches the maximum brazing temperature. With continued heating to brazing temperature, the gap between the tapers opens to an optimal amount for brazing such as 0.002xe2x80x3. The steps ensure that the active tube length is maintained with precision because it is independent of machining tolerances of the tapers. This embodiment also allows for a predetermined braze gap.
Cooling of the brazed assembly, for both embodiments, results in the balance bar attempting to contract more than the titanium flow tube and the titanium connecting ring. The balance bar""s greater radial contraction is opposed by the connecting rings which have moved into and been brazed to the tapers of the balance bar. This contraction results in the braze joints between the balance bar and the connecting rings being put in compression. The compression of the connecting ring by the balance bar also puts the braze joints between the connecting rings and the flow tube in compression. The compression results in stronger braze joints between the balance bar, the connecting rings, and the flow tube.
In accordance with yet another alternative embodiment of the invention, a configuration is provided that can compensate for the thermal expansion difference between the connecting ring and the balance bar and also the thermal expansion difference between the connecting ring and the flow tube. This configuration is used when the connecting rings are made of a third material that has an expansion coefficient that is between that of the flow tube and that of the balance bar. The use of a connecting ring of a material with a coefficient of expansion between those of the flow tube and the balance bar has the advantages of equalizing the thermal stress on the various elements and reducing the peak thermal stress accordingly when the brazed elements are cooled.
This third embodiment utilizes an additional titanium ring, called a tube ring, that encircles and is brazed to the titanium flow tube. This tube ring has a tapered outer diameter that matches a tapered inner diameter of the connecting ring. The connecting ring also has a tapered outer diameter that corresponds to a tapered inner diameter of the balance bar as in the prior embodiments.
The tapers on the inside and outside of the connecting ring are related by the differences in the thermal coefficient of expansion. The relationship between the tapers is necessary because the connecting rings can only move axially a single distance for both the internal and external tapers. If the difference in the expansion coefficients between the connecting ring and the tube is equal to the difference in expansion coefficients between the connecting ring and the balance bar, then the inner and outer taper angles can be equal. If the difference in the expansion coefficient between the balance bar and the connecting ring is larger than the coefficient difference between the connecting ring and the flow tube, the outer taper angle of the connecting ring will be larger than the inner taper angle of the connecting ring. The inner and outer taper angles of the connecting ring are designed so that at brazing temperature both the braze gaps are an optimal size. This embodiment can also have steps at the inner ends of the balance bar taper to better control the active tube length and the braze gaps.
In accordance with the present invention, the assembly comprising a flow tube, connecting rings surrounding the flow tube, and a surrounding balance bar is vertically oriented and brazed in accordance with the present invention by inserting one end of the vertically oriented flow tube together with the tapered connecting rings and balance bar into a support base. This base supports the assembly by the lower connecting ring. A weight having a center opening adapted to receive the upper end of the flow tube is placed on the upper connecting ring. The entire structure including the base support, the flowmeter assembly, and the top weight are placed in a brazing oven with braze material applied to the joints that are to be brazed. The entire assembly is heated to brazing temperature at which the stainless steel balance bar expands far more than does the connecting ring and flow tube. This expansion permits the weight on the top end of the assembly to press the tapered connecting rings into the balance bar by the required amount. The braze material flows into the surfaces to be joined. When cooled, the surfaces comprising the junction of the inner surface of the balance bar and the outer surface of the connecting ring as well as the surfaces comprising the junction of the inner surface of the connecting ring and the outer surface of the flow tube (and/or the tube ring) are now bonded. Also, the brazed surfaces are held together by the compressive forces generated by the stainless steel balance bar whose higher thermal coefficient of expansion attempts to compress radially the titanium connecting ring and flow tube. The connecting ring and flow tube generate forces that oppose the compressive force generated by the stainless steel.
It can therefore be seen that the method and apparatus of the present invention achieves an advance in the art by providing a simple and inexpensive brazing of a flow tube, connecting ring, and a balance bar made of dissimilar material.
An aspect of the invention comprises a Coriolis flowmeter having:
a flow tube;
connecting ring means having a center opening through which said flow tube extends;
a tubular balance bar coaxial with said flow tube and surrounding an axial portion of said flow tube;
said balance bar has a greater thermal coefficient of expansion than does said flow tube;
axial end portions of said balance bar coaxial with and surrounding at least a portion of said connecting ring means;
a radial inner circumferential surface of said connecting ring means coupled to an outer circumferential surface of said flow tube;
a radial outer circumferential surface of said connecting ring means is tapered with a decreasing radius in a first direction with respect to the axial center of said flow tube;
a radial inner circumferential surface of said end portions of said balance bar has a taper that matches said taper of said connecting ring means;
said connecting ring means is adapted to be partially inserted into ends of said balance bar prior to a brazing operation and then fully inserted into said balance bar, during said brazing operation as said balance bar expands in diameter more than said connecting ring;
said tapered inner circumferential surface of said end portions of balance bar is adapted to be affixed by braze material to said tapered outer circumferential surface of said connecting ring means at the termination of said brazing operation.
Another aspect is that said connecting ring means comprises:
a first connecting ring and a second connecting ring each adapted to be brazed to a different end of said balance bar;
a tapered radial outer circumferential surface of said first connecting ring is adapted to be brazed to said tapered radial inner circumferential surface of a first end of said balance bar during said brazing;
a tapered radial outer circumferential surface of said second connecting ring is adapted to be brazed to said tapered inner circumferential surface of a second end of said balance bar during said brazing operation.
Another aspect is that said first direction defines a taper that has a decreasing radius towards the axial center of said flow tube.
Another aspect is that said first direction defines a taper that has an increasing radius towards the axial center of said flow tube.
Another aspect is that said balance bar comprises a first and a second balance bar segments having axial inner end portions coupled to each other by spring means to accommodate a differential axial coefficient of expansion between said flow tube and said balance bar.
Another aspect is that said balance bar comprises an integral elongated member.
Another aspect is that said radial outer circumferential surface of said flow tube is affixed by braze material to said radial inner circumferential surfaces of said first and second connecting rings.
Another aspect is that said balance bar has a thermal coefficient of expansion greater than that of said first and second connecting rings and that said first and second connecting rings have a coefficient of expansion equal to that of said flow tube.
Another aspect includes a step on said radial inner end of said tapered circumferential surface of said balance bar that engages an axial inner end of said first and second connecting rings to limit the amount by which said first and second connecting rings can be axially inserted into said balance bar.
Another aspect comprises first and second annular tube rings coaxial with and encircling axial portions of said flow tube;
said axial portions of said radial outer circumferential surface of said flow tube are affixed by braze material to radial inner circumferential surfaces of said first and second annular tube rings;
a radial outer circumferential surface of each of said first and second annular tube rings is tapered with an increasing radius towards said axial center of said flow tube;
said radial inner circumferential surface of said first and second connecting rings has a taper that matches the taper of said first and second annular tube rings and has a diameter that decreases radially towards the axial center of said balance bar,
said radial inner circumferential surface of said first and second connecting rings is adapted to be affixed by braze material to said radial outer circumferential surface of said first and second annular tube rings during said brazing operation.
Another aspect includes a step on said radial inner circumferential surface of said balance bar that engages the axial inner end of said first and second connecting rings to limit the amount by which said first and second connecting rings can be axially inserted into said balance bar during said brazing operation.
Another aspect is that said balance bar has a thermal coefficient of expansion greater than that of said first and second connecting rings and that said first and second connecting rings have a thermal coefficient of expansion greater than that of said first and second annular tube rings and that of said flow tube.
Another aspect is that said first and second connecting rings and said first and second annular tube rings and said balance bar have different thermal coefficients of expansion.
Another aspect comprises a method of assembling a Coriolis flowmeter having a flow tube, a connecting ring means, and a tubular balance bar, said method comprising the steps of:
extending said flow tube through a center opening in said tubular balance bar;
positioning said flow tube so that said flow tube extends through a center opening of said connecting ring means and is coaxial with said balance bar;
positioning said connecting ring means so that axial end portions of said balance bar are coaxial with and surround at least a portion of said connecting ring means;
coupling a radial inner circumferential surface of said connecting ring means to said flow tube;
a radial outer circumferential surface of said connecting ring means is tapered in a first direction with a decreasing radius with respect to the axial center of said flow tube;
radial inner circumferential surfaces of said axial end portions of said balance bar have a taper that matches said taper of said connecting ring means;
said balance bar has a greater thermal coefficient of expansion than does said flow tube; and
brazing said tapered inner circumferential surface of said end portions of balance bar to said tapered outer circumferential surface of said connecting ring means, said connecting ring means being axially moved within said balance bar during said brazing beyond the axial location within said balance bar prior to brazing.
Another aspect is that said connecting ring means comprises a first connecting ring and a second connecting ring; said step of brazing comprises the step of moving each said connecting ring in an axial direction during said brazing operation so that the radial pressure exerted on said flow tube by each said connecting ring increased subsequent to brazing.
Another aspect is that said first direction defines a taper of said connecting ring having a decreasing radius towards the axial center of said flow tube.
Another aspect is that said step of brazing comprises the steps of:
axially moving said first and second connecting rings towards said axial center of said balance bar during said step of brazing; and
cooling said brazed surfaces so that said greater thermal coefficient of expansion of said balance bargenerates a radially compressive force against said first and second connecting rings and said flow tube.
Another aspect is that said step of bonding includes the step of brazing said outer circumferential surface of said flow tube to said inner circumferential surface of said first and second connecting rings; and
axially moving said first and second connecting rings towards said axial center of said balance bar during said step of brazing.
Another aspect includes the step of forming a step on said inner tapered circumferential surface of said balance bar that engages axial inner ends of said first and second connecting rings to limit the amount by which said first and second connecting rings can be axially inserted into said balance bar during said brazing.
Another aspect is that:
first and second annular tube rings couple said flow tube with said first and second connecting rings; said method further includes the step of:
bonding said outer radial circumferential surface of said flow tube to an inner radial circumferential surface of each of said first and second annular tube rings;
an outer radial circumferential surface of said first and second annular tube rings is tapered to have an axially increasing radius towards said axial center of said flow tube;
said inner radial circumferential surface of said first and second connecting rings have a taper that matches that of said first and second annular tube rings and has a radius that decreases towards the axial mid portion of said balance bar, and
brazing said tapered inner circumferential surface of said first and second connecting rings to said tapered outer circumferential surface of said first and second annular tube rings.
Another aspect is that said step of axially moving said first and second connecting rings towards said axial center of said balance bar during said step of brazing.
Another aspect includes the step of forming a step on said inner circumferential tapered surface of said balance bar that engages the axial inner end of said first and second connecting rings to limit the amount by which said first and second connecting rings can be axially inserted into said balance bar during said step of brazing.
Another aspect is that said balance bar has a thermal coefficient of expansion greater than that of said first and second connecting rings and that said first and second connecting rings have a thermal coefficient of expansion greater than that of said first and second annular tube rings and that of said flow tube;
said method further includes the step of axially moving said first and second connecting rings towards said axial center of said balance bar during said step of brazing.
Another aspect includes the steps of:
orienting said flow tube and said balance bar so that a first end of said flow tube extends into a recess of a base;
placing said first and second connecting rings concentric with said flow tube and axially at least partially within first and second ends of said balance bar so that the outer ends of said connecting rings extend axially beyond the ends of said balance bar;
placing braze material proximate the axial end extremities of the junctions of surfaces common to said balance bar and said first and second connecting rings and junctions of surfaces common to said connecting rings and said flow tube;
placing a mass on a second end of said flow tube so that said mass exerts a force on said connecting rings urging them axially into engagement with said balance bar; the outer ends of said connecting rings then extending axially beyond the ends of said balance bar;
heating said balance bar and said connecting rings and said flow tube to brazing temperatures;
the brazing temperature being effective to expand said balance bar radially to enable said connecting rings to move axially inward within said balance bar; and
cooling said brazed surfaces so that said greater thermal coefficient of expansion of said balance bargenerates a radially compressive force against said first and second connecting rings and said flow tube.
Another aspect is that said balance bar comprises first and a second axially separated segments and that said method further comprises the steps of connecting spring means between the axial inner end of each of said balance bar segments to accommodate a differential thermal coefficient of expansion between said flow tube and said balance bar segments.
Another aspect is that said first direction defines a taper of said connecting rings having an increasing radius towards the axial center of said flow tube.
Another aspect further comprises the steps of:
axially moving said first and second end portions of said connecting rings towards said axial center of said balance bar during said step of brazing;
brazing said tapered inner circumferential surfaces of said first and second end portions of balance bar with said tapered outer radial circumferential surfaces of first and second said connecting rings; and
cooling said brazed surfaces so that said greater thermal coefficient of expansion of said balance bar generates a radially compressive force against said first and second connecting rings and said flow tube.
Another aspect is that said balance bar comprises a pair of axially separated segments and that said method further includes the steps of:
extending a first end of said flow tube through a center opening of said first connecting ring;
extending a second end of said flow tube through a center opening of a second connecting ring;
affixing said first and second connecting rings to said flow tube;
extending said first end of said flow tube and said first connecting ring through a first balance bar segment;
extending said second end of said flow tube and said second connecting ring through a said second balance bar segment;
placing braze material proximate the axial extremities said first and second connecting rings proximate said flow tube and said balance bar segments;
exerting a force on said balance bar segments urging them towards an axial center of flow tube and said balance bar;
heating said balance bar segments and said connecting rings and said flow tube to brazing temperatures;
the brazing temperature being effective to expand said balance bar radially to enable said balance bar end segments to move axially inward toward said axial center of said flow tube and said balance bar; and,
cooling said brazed surfaces so that said greater thermal coefficient of expansion of said balance bar segments generates a radially compressive force against said first and second connecting rings and said flow tube.
Another aspect is a method of assembling said Coriolis flowmeter, said method comprising the steps of:
extending said flow tube through a center opening in said tubular balance bar;
positioning said connecting ring means so that axial end portions of said balance bar are coaxial with and surround at least a portion of said connecting ring means;
positioning said flow tube so that said flow tube extends through a center opening of said connecting ring means and is coaxial with said balance bar;
positioning a radial inner circumferential surface of said connecting ring means against said flow tube;
said outer radial circumferential surface of said connecting ring means is tapered in a first direction with a decreasing radius with respect to the axial center of said flow tube;
said inner radial circumferential surfaces of said axial end portions of said balance bar have a taper that matches said taper of said connecting ring means; and
brazing said tapered inner circumferential surface of said end portions of balance bar to said tapered outer circumferential surface of said connecting ring means; and
moving said connecting ring in an axial direction during said brazing operation so that the radial pressure exerted on said flow tube by said connecting increases subsequent to brazing.